1. The Field of the Invention
The present invention relates generally to thermally enhanced oil recovery and more specifically to a method and apparatus for accurately profiling the steam flow through the formations of a field.
2. Background of the Invention
It has often been the case that crude oil, in its natural state, is sufficiently viscous as to require the injection of means, such as surfactants or heat, into the reservoir in order to increase the fluidity of the crude oil and drive it to a production well where it can be recovered. This is often done by the injection of steam into a plurality of injection wells in a spaced array in the reservoir. Ideally the reservoir would be completely homogeneous and the steam entering from each injection well would flow evenly to all portions of the reservoir and drive all of the crude oil to recovery wells. However, it is the actual case that the reservoir is formed by a plurality of pockets of crude oil in formations which can be extensive in that they span a large area intercepting both injection and recovery or production wells. These formations can be discontinuous, at certain levels, and they can also be interconnected by fissures or fractions. Steam injected into a formation at a first level can actually arrive at the production well from a completely different formation at a different, usually shallower, second level. Normally the steam, which is less dense than the in situ crude and other fluids within the formation, will be rising as it moves outwardly through the field from the injection well. Eventually steam breakthrough will occur, which means the steam will have formed at least one path between an injection well and at least one production well. Any subsequently injected steam will follow this path which will be the path of least resistance between the injection and production wells. This path may override and/or bypass large portions of the petroleum reservoir in that particular formation.
There have been a number of methods and devices proposed for monitoring the injection of steam into an injection well with the monitoring determining both the quality and the quantity of the steam that is being injected and the rate of steam flow. It is even possible, with the use of proper tools, to confine the steam injection to a specific zone of the injection well so that the level at which the steam is entering the formation is known. However, it is highly unlikely that the steam will arrive at that same level at a production well because of the tendency for steam to rise as it moves outwardly from the injection well and geological faults or fissures which may be present within the reservoir.
The steam may cross formation boundaries and not stay in the originally injected formation for any of a number of reasons. For example, the injected formation may pinch out or meet a permeability barrier between the injection and production wells. All oil bearing zones penetrated by the production well may not exist at the injection well and vice versa. Any of these might cause the steam to arrive at the production well in a formation different from the one into which it was originally injected.
In the above-noted methods and apparatus for measuring the steam movement, it has been known to use radioactive tracers. For example, krypton (85) has been used to trace the vapor phase and sodium iodide (131) has been used to trace the liquid phase of steam. In either case the isotope is generally injected into the steam line between the injection well and the steam generator and moves down the well tubing with the steam until it reaches the formation where the isotope remains near the face of the formation for a brief period. One measurement method then runs a typical gamma ray log immediately following the tracer injection and the recorded gamma ray intensity at any point in the well is then assumed to be proportional to the amount of steam injected into the formation at that point. Another measuring method employs a downhole detector that senses the tracer as it goes by. By placing detectors at various points between formations, the amount of steam going into each formation can be deduced.
However, very little, if any, effort has been made to determine where the steam comes out of a formation at the production well and what the source of that steam may be. If the level at which the steam enters the production well is not known, then it would not be possible to make an accurate determination as to where override might have occurred or if, in fact, there were pockets of petroleum in formations which were not being contacted by this steam at all. If there is a determination as to the level at which the steam is entering the production well, then it would be possible, for example, by blocking off release of steam at the surface of the production well, to force the steam to flow back into formations which had escaped the initial or original steam flow.